[How science is done: Observations - natural history; Classification and laws; Hypotheses; Theories]

In contrast to how this subject is presented in the text, we will discuss the development of the theory of plate tectonics by looking at how the theory developed historically - an example of how science is actually done!

Continental Drift

Emphasized the lateral movements in the Earth's crust

Alfred Wegener in 1915

Geographic fit of continental margins - an idea that goes back to philosopher Francis Bacon in the 17th century; s

Match of geological features: mountain belts, Paleozoic glacial deposits, fossils. Wegener compiled this evidence. In hindsight quite compelling.

Wegeners ideas not taken seriousky by many geologists; much resistance to his ideas during the 1920's and 30's, especially in N. America!! Idea of continents as boats ploughing through solid rock not palatable.

Paleomagnetism

TRM, DRM, inclination and paleolatitude, declination and rotation

Polar wander curves - continents moved with respect to N magnetic pole

Converging polar wander curves - evidence for drift & relative motion of continents

Sea floor spreading
how the oceans are created

Marine geology investigations during 1950's and early 1960's

Morphology of ocean basins: developed as a result of SONAR submarine hunting expeditions in WW II; found more ocean floor than submarines!

Found: mid-ocean ridges, fracture zones, abyssal ocean floor, trenches and arcs

Heat flow patterns over mid ocean ridge

Harry H. Hess, professor of Geology at Princeton proposed expansion of ocean floor (1960)

Magnetic striping & Vine-Matthews-Morley hypothesis

magma is injected at mis ocean ridge - as cools takes on magnetic field polarity that exists at the time

Transform faults and fracture zones - J. Tuzo Wilson

Age of ocean floor - can be determined by looking at stripes

Subduction zones
where the ocean floor reterns to the mantle

Seismic patterns: global distribution,

Wadati-Benioff zone:

Explanation - isotherms in cold (& hence dense) descending slab.

Putting it all together - Plate Tectonics

Idea of plates: redefine lithosphere

Plate Boundaries:

Divergent

Convergent

Transform

[Euler poles]

Rate of plate movements - cm per year

In recent yearsm, relative plate movement confirmed directly using observations of Global Positioning System - GPS

Interaction of mantle and plates: Hot spots

Hawaiian - Emperor ocean island/seamount change

Idea is that hotspots have to be fixed in mantle, therefore hot spot tracks give motion relative to mantle

Large igneous provinces, ocean plateaus and hot spots

Oceanic: Caribbean, Ontong Java,

Continental: Deccan, Paraná, Siberia, Columbia River

What drives the plates?

basal drag of convecting mantle?? no!!

Slab pull,

ridge push